1. Field of the Invention
The present invention is directed to an x-ray diagnostics installation, and in particular to such an installation which produces an image having a dominant region of primary medical interest.
2. Description of the Prior Art
It is known from German OS 31 27 648 to provide an x-ray diagnostics installation with a detector which measures the mean image brightness of a dominant region of the light image at the output screen of the x-ray image intensifier. The installation can produce video images as well as individual images and series of individual images. The detector serves the purpose of maintaining the mean image brightness constant in the dominant region. In German OS 31 27 648 this is accomplished by disposing a detector in the parallel beam path of the light between the output luminescent screen of the x-ray image intensifier and the video camera. The dominant region is selected from the total light image and is coupled out of the parallel beam path by a mirror. Selection of the dominant image can be undertaken with a dominant disk disposed in front of a photomultiplier or an element having a plurality of light-sensitive surfaces which are individually selectable. The adjustment of the shape, position and size of the dominant region using such a dominant disk, however, is limited by the size of the disk.
It is generally standard in such x-ray image intensifiers to switch the format of the x-ray image intensifier dependent on the size of the subject to be observed, and dependent on a desired magnification, so that identical surfaces on the input luminescent screen illuminate or correspond to a different surface on the output luminescent screen. A magnification of the image of the examination subject can thus be achieved in a simple manner.
A disadvantage is such known systems is that the dominant region changes due to changing the format of the x-ray image intensifier. This can be prevented by providing an appropriate opening in the dominant disk for each available adjustment of the x-ray image intensifier, with the opening in the dominant disk then being adjusted. This means that a plurality of openings corresponding to the imaging scale of the x-ray image intensifier must be provided for each desired shape and position of the dominant region, so that the dominant disk becomes enlarged to an undesirable extent and accordingly becomes cumbersome. If a detector having a plurality of light sensitive elements or surfaces were used, the electrical connections to the surfaces could be modified in accordance with the desired change in shape and position, however, the resolution with respect to the examination subject in the x-ray image undesirably changes due to the modification of the imaging scale of the x-ray image intensifier. Such a detector array would therefore have to be extremely finally sub-divided in order to be able to achieve an optimum matching of the detector surfaces to the examination subject, given a modification of the imaging scale of the x-ray image intensifier.